Rain, sleet and snow having always presented a vision problem for the driver of a moving vehicle. The windshield wiper blade has attempted to minimize the problem by clearing the windshield of the light obstructing moisture. Such blades are typically formed of rubber or rubber-like materials. Over the years, wiper blades have been modified in many ways in order to enhance wipe quality and therefore visibility during precipitation. In some instances, the configuration of the blade has been changed to give a plurality of contact surfaces on the blade. Various modifications have been introduced to improve the consistency and integrity of the wiping edge.
Workers in the wiper field have made prior attempts to use silicone rubber as a base for a wiper blade composition, but without notable success. This is because as a material, silicone rubber is superior to natural or other synthetic rubbers for several reasons. Silicone rubber, i.e., high molecular weight, vulcanizable polydiorganosiloxane, is able to withstand wide temperature variations without an appreciable effect on its physical properties. Further, silicone rubber is virtually unaffected by ultraviolet radiation, even over long periods of time. It is also resistant to ozone, oil, salt, water and other road and automotive chemicals. If silicone rubber compositions could be used for wipers, they would have the ability to resist compression set and withstand solar radiation, chemicals, ozone, water, oil and salt without adverse effects. Thus, if commercially manufacturable silicone wipers were possible, such wipers would have a much longer life than conventional natural rubber wipers now in general use.
Silicone rubber as used for wiper compositions has had one significant drawback: it has an unacceptably high coefficient of friction with respect to glass. In preliminary testing by at least one worker in the field, silicone wipers exhibited such a high coefficient of friction that they tore loose from the wiper arms instead of wiping the windshield. Less catastrophic effects of this high coefficient of friction include an unacceptably loud squeak or chatter as the wiper traverses the windshield, and unacceptably high loads on the windshield wiper motor.
U.S. Pat. No. 4,904,434 (hereinafter, "the '434 Patent") and U.S. Pat. No. 4,981,637 (hereinafter, "the '637 Patent"), both to Hyer (together, the "Hyer Patents"), disclose a method of forming an improved wiper blade utilizing an extrusion process. In this method, a continuous length of curable elastomer is extruded through a die having a die opening which is shaped to produce a pair of wiper blades joined at a mid-section thereof in edge-to-edge relation. The '434 Patent discloses a method of scoring the mid-section of the continuous length of the extruded elastomer and the '637 Patent teaches preforming the mid-section to produce a weakened mid-section. In both cases, the extruded elastomer is then cured and, thereafter, the length is separated into two separate lengths of wiper blade by dividing the length either along the score line (the '434 Patent) or along the preformed mid-section (the '637 Patent).
In addition to the disclosure of a method of forming a wiper blade using an extrusion process, the Hyer patents disclose a silicone rubber composition for such wiper blades. These compositions are disclosed as containing about 100 parts silicone polymer, about 40 parts filler, from about 0.5 to 2.0 parts vulcanizing and 0 to 10 parts of other enhancement additives. The polysiloxane polymers are further disclosed as having methyl and vinyl organic groups, with the vinyl level being in the range of 0 to 5 mole percent. The fillers are specified as including fumed silica, precipitated silica, ground quartz, calcium carbonate and iron oxide. The Hyer patents fail to specify the relative amounts of these different filler components.
One conventional method to measure the amount of silica filler in a wiper blade is its specific gravity. It has been found that the Hyer wiper blades above described typically have a specific gravity of 1.1 to 1.2. Such wiper blades still exhibit a relatively high coefficient of friction with respect to glass, making them less than optimally suited for commercial use.
U.S. Pat. No. 3,972,850 to Hamilton (hereinafter "Hamilton") discloses a windshield wiper blade comprising silicone elastomer, dispersed filler and an additive which imparts lubricant and hydrophobic characteristics to a windshield. The polymerized silicone elastomer in Hamilton comprises approximately 3 to 20 parts dispersed filler selected from silica, mica, molybdenum disulfide and tetrafluoroethylene, and approximately 0.2 to 10 parts of either dicoco dimethyl ammonium chloride or a di[(n-hydroxy)polydimethyl-siloxanyl]n-(diethylamino) methylsilane copolymer. The Hamilton reference states that amounts of silica filler over 20 parts by weight are unsatisfactory for wiper blades.
While Hamilton discloses a composite silicone rubber material capable of imparting hydrophobic properties to windshields, Hamilton's composition is not economically feasible for mass manufacture due to Hamilton's use of exotic and expensive additives. Additionally, Hamilton uses an expensive molding technique to form the wiper blade. Further, Hamilton does not teach a wiper blade physical structure. The surface area of contact with the windshield, and therefore the profile of the wiper blade, must vary as the composition of the blade is varied in order to yield the same frictional force when the wiper blade is applied to glass.
In view of these prior art attempts, a long felt need continues to exist for an improved windshield wiper blade that is capable of inexpensive manufacture; that has an improved edge that provides a clean and unobstructed view; that has low temperature flexibility providing easy release from ice collecting on the blade; that has improved weatherability providing a surface impervious to ultraviolet and ozone decay and unaffected by environmental temperature extremes; that has hydrophobic properties; that has increased wiper durability; and that has resistance to salt and acid rain corrosion. A need has also existed in the industry for wiper blades capable of accepting non-black colorants. Conventional, natural rubber wiper blades are colored black in order to protect the wiper blades from harmful ultraviolet radiation. Windshield wiper blades having light or bright colors have not therefore been commercially acceptable in the past.